add note about knitr

Co-authored-by: Michael Chirico <michaelchirico4@gmail.com>

.gitlab-ci.yml

@@ -212,6 +212,8 @@ test-lin-ancient-cran:
   script:
     - *install-deps
     - Rscript -e 'install.packages("https://cran.r-project.org/src/contrib/Archive/xts/xts_0.12.2.tar.gz", repos = NULL, type = "source")'
+    # knitr requires evaluate, which requires R 3.6.0.
+    # Restore checking vignettes if upgrading our R dependency means knitr can be installed.
     - R CMD check --no-manual --no-build-vignettes --ignore-vignettes $(ls -1t data.table_*.tar.gz | head -n 1)
 
 .test-win-template: &test-win